Conventionally, two ramp signals are used to control the duty cycle of the switches in the converter, one for buck operation and one for boost operation. Comparators compare the output signal of an error amplifier with the ramp signals to generate the pulse width modulation signal for controlling the switches in the converter stage. In buck operation, the input signal is higher than the output signal. In boost operation, the input signal is lower than the output signal. When the input voltage is in a range close to the output voltage, a change between buck and boost mode occurs. The change between buck and boost mode involves a discontinuity in the transfer function at the transition point. A gap between the ramp signals would result in an interruption of the pulse width modulation, and unpredictable low frequency noise would occur.
To avoid this problem, conventional approaches provide a wide overlap between the two ramp signals. Due to the overlap between the ramp signals, the switches in the converter power stage are operated in both of the buck mode and the boost mode. In the overlap region, the converter operates in a buck-boost mode. With a wide overlap, buck-boost operation results over a wide range of supply voltage. While the buck-boost operation avoids the discontinuity problem, this is achieved at the expense of efficiency. Due to the actuation of all the switches in the power stage in each clock period, a lot of energy is lost by dissipation. The result is an efficiency curve with a dip in the range where the supply voltage is close to the output voltage. Since this is a condition in which the converter is operated most of the time, the reduced efficiency in buck-boost operation is a major drawback.